This invention relates to ceramic arc tubes and more particularly to ceramic arc tube assemblies and methods of making such assemblies.
Over the years ceramic arc tubes composed of materials such as polycrystalline alumina have been used to contain the discharges of high-pressure sodium lamps. This has resulted in the development of a number of arc tube configurations for use in sodium lamps. For example, U.S. Pat. No. 4,766,347 describes a three-piece arc tube configuration wherein the arc tube comprises a ceramic body with tubular closure members. The closure members which receive the electrodes are sealed directly in the ends of the ceramic body. In another example, U.S. Pat. No. 5,426,343 describes a three-piece arc tube configuration wherein end-sealing buttons are used which have an extending electrode-receiving member integral therewith.
More recently, ceramic arc tubes have been employed in metal halide lamps. For example, U.S. Pat. No. 5,424,609 describes a five-piece ceramic arc tube configuration for a metal halide lamp. The five-piece arc tube includes a cylindrical body, a pair of end-sealing buttons, and a pair of capillary tubes sealed to the buttons. The manufacture of these ceramic arc tubes requires extrusion or pressing of the individual components, as well as multiple assembly and heat treatment steps. These multiple steps result in increased handling which increases the manufacturing cost.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide a ceramic arc tube assembly which facilitates and simplifies the manufacturing of ceramic arc tubes.
It is another object of the invention to provide a method for making ceramic arc tubes which reduces the number of handling and firing steps in the manufacturing process.
In accordance with one object the invention, there is provided a ceramic arc tube assembly comprising a hollow body having at least one open end containing an end cap, the end cap having a capillary tube and a transient assembly button, the capillary tube extending outwardly from the hollow body and having a length inserted into the open end and forming a seal with the hollow body, the transient assembly button being fixed around the capillary tube and contacting an edge of the open end, the interaction between transient assembly button and the edge of the open end determining the length of the capillary tube inserted into the open end, and the transient assembly button being capable of removal from the capillary tube without causing damage to the arc tube assembly.
In accordance with another object of the invention, there is provided a method of making a ceramic arc tube comprising the steps of:
(a) fixing a transient assembly button around a capillary tube to form an end cap;
(b) inserting the end cap into an open end of a hollow arc tube body until the transient assembly button contacts an edge of the open end;
(c) heating the assembly to form a mechanical seal between the capillary tube and the open end of the hollow body;
(d) removing the transient assembly button without damaging the assembly; and
(e) sintering the assembly to form the ceramic arc tube.